1. Technical Field
The present invention relates to a technique for estimating a state of charge of a secondary battery, such as a nickel-metalhydride battery and a lithium ion battery.
2. Related Art
A technique for estimating a state of charge (hereinafter abbreviated as “SOC”) of a nickel-metalhydride battery or a lithium ion battery and controlling charging and discharging of the secondary battery on the basis of the estimated SOC has hitherto been known. In particular, a hybrid electric vehicle using a secondary battery as a power source for a drive motor, and the like, performs travel control on the basis of the SOC of the secondary battery and therefore requires highly-accurate estimation of the SOC.
A method for estimating an SOC by measuring an integrated current value input by or output from the secondary battery, a method for estimating an SOC from correlation between a preset open circuit voltage (OCV) and an SOC, a method for estimating an SOC by combination of the methods, and the like, have hitherto been proposed. However, as is well known, electromotive force of the secondary battery fluctuates by reason of a memory effect. Even when an SOC is estimated from the correlation between an OCV and an SOC achieved in an initial state, the accuracy of the thus-estimated SOC deteriorates. For this reason, making a correction to the correlation according to the memory effect has been put forward.
2004-22322 A describes a technique for storing in memory a range of an SOC and a range of OCV where a correction value is applied in response to a memory effect; computing a correction value on the basis of the temperature of a battery and a charge/discharge current; applying the correction value only within the range of the SOC and the range of the OCV stored in memory, to thus prepare an OCV-SOC map; and estimating an SOC from the OCV-SOC map.
FIG. 10 shows an OCV-SOC map achieved in an initial state and after production of a memory effect. In the drawing, a horizontal axis represents an SOC (%), and a vertical axis represents an OCV (V). Correlation was achieved when charge and discharge operations were repeatedly performed in the vicinity of the center level of the SOC. A voltage drop is in a discharge side, and a voltage rise is in a charge side.
However, even when a correction value is computed from the temperature of the battery and the charge/discharge current, performance of accurate correction is not necessarily assured, which raises a problem of a failure to enhance the accuracy of estimation of the SOC.